1. Field of the Invention
The present invention pertains to pigments. More particularly, the present invention concerns methods for improving the dispersibility of pigments. Even more particularly, the present invention concerns methods for coating pigments to improve the dispersibility thereof.
2. Prior Art
Conventionally, water-based pigments are produced by precipitating water-soluble salts followed by the filtration, washing and, then, drying of the salts. During water removal the pigment particles typically undergo aggregation or agglomeration. Because of its higher surface tension, compared to the pigment, water brings the pigment particles to the closest possible distance with a maximum of force. This aggregated pigment is subsequently hard to disperse since any organic binder or solvent is unable to wet and separate the primary particles of agglomerates due to van der Waals attractive forces. Where the ultimate properties of a pigment are dependent on its final state of subdivision, considerable mechanical energy is required in order to achieve the desired state of fineness. Where pigment particles are needle shaped, as in transparent oxide, the particle-to-particle contact occurs on the longer axis, making separation even more energy intensive. For transparent pigments, where the ultimate division has to be smaller than the wavelengths of visible light, great effort is required to disperse the pigment.
In order to improve the dispersibility of highly agglomerated pigment, many types of wetting, dispersing, flowing agents or surfactants are used. They are added to the binder-solvent system while the pigment is being treated or oftentimes while the pigment is manufactured. In manufacturing coatings and inks a multitude of binders and solvents are used. Generally, polymeric binders such as acrylic polymer, alkyd polyester, nitrocellulose, polyamide, polyvinyl, polystyrene and the like are used. In order to produce a durable coating the binder polymer has to interact with the surface of the pigment. Aside from the physical accessibility of the pigment surface certain considerations of compatibility of chemical groups are important. Thus, the present art relies on a multitude of surfactants or wetting agents to establish a pigment surface that is compatible with a given binder. Hence, no universal treatment is available wherein the pigment surface is compatible in all types of binder polymers.